BEIJING, China (CNN) -- Unregistered churches are attracting millions of worshippers in China, exposing an enduring rift between the government and the Vatican.

China broke off relations with the Vatican under Chairman Mao but over the past couple of years ties seemed to be warming up.

A state-controlled church is popular, but the underground parishioners say it is not the real church. From the state's perspective, these believers are loyal to the pope, not the Communist Party.

In 2007, Pope Benedict sent a letter to underground Catholics in China, encouraging them to reunite with the official church. But a central conflict remains -- the Pope maintains the right to appoint bishops, a right China maintains belongs to the government.

And although the underground churches operate with little interference, persecution does persist.